Incidence of overweight and obesity continues to increase and, in the US, normal weight individuals are now a minority of the population. The current epidemic of obesity appears to result from environmental factors inactivating mechanisms that regulate energy balance. There is unequivocal evidence that peripheral administration of leptin, an adipose tissue-derived hormone, specifically reduces body fat content in both experimental animals and obese humans on a weight reducing diet. We will test the hypothesis that peripheral infusions of physiological doses of leptin reduce body fat through a variety of mechanisms and that consumption of a high-fat diet or development of obesity, independent of diet composition, inhibits some, or all of these mechanisms to induce a state of leptin resistance facilitating the progression of obesity. Here we define leptin resistance as a failure of leptin to reduce body fat content. Specific Aim One will determine how leptin reduces body fat content in low-fat fed mice, measuring rates of lipid synthesis and breakdown, and the levels of enzymes involved in adipocyte lipid metabolism. We will test whether leptin acts directly on adipocyte metabolism, or indirectly through neural input to the tissue or by modifying the release of other metabolically active hormones. We will then test which of these mechanisms is inactivated either by consumption of a high-fat diet that does not induce obesity or by consumption of a high-sucrose diet that does induce obesity. Specific Aim Two will investigate whether leptin reduces body fat content by changing the number of adipocytes present. In vivo and in vitro studies will test the effect of peripheral infusions of leptin on adipocyte proliferation, differentiation and apoptosis and whether leptin resistance in high-fat fed or obese mice is due to an inhibition of leptin action on adipocyte development or apoptosis. Preliminary studies show that db/db mice that are deficient in the long-form leptin receptor are less obese than db/db mice that are deficient in all membrane-bound leptin receptors, possibly due to inhibition of preadipocyte proliferation. Therefore, we will use these two strains of db/db mice to test the importance of short-form leptin receptors in the regulation of adipocyte development and apoptosis. Thus, successful completion of the proposed studies will demonstrate how high-fat diets or obesity inactivate one of the physiological systems that normally regulate energy balance, providing new opportunities for development of new strategies for the treatment, or prevention, of obesity.